1. Field of the Invention
The present invention relates an improved method and apparatus for demodulating a telemetry signal, i.e., recognizing or identifying digital information in an analog signal. Specifically it pertains to drilling telemetric systems in formation evaluation or borehole telemetry through noisy transmission channels.
2. Description of the Related Art
In the development, completion, and operation of natural hydrocarbon reservoirs, various telemetric systems and techniques are known and employed to achieve what is known in the art as measurement while drilling (MWD).
For the purpose of this application, MWD includes any type of data transmission from sensor units in the drill bit, bottom hole assembly, or any other part of the sub-surface drill string. Another acronym often encountered in the art besides MWD is LWD (Logging While Drilling). MWD includes in particular low data bit rate transmission systems, as operating below 10 KHz, preferably below 1 KHz, such as acoustic telemetry through the drill string itself, mud pulse or electro-magnetic telemetry. For the scope of the present invention however, the technological field can be better characterized by the ratio of the speed of data processing on the receiver side and the transmission rate. The computing speed is measured in floating point operations per second (flops). Thus, the invention is preferably operable for telemetry processing above 3*10.sup.5 flop/bit, more preferably above 4*10.sup.5 flop/bit.
In the currently prevailing techniques data are transmitted by means of a mud pressure pulse generator located either inside or being part of the drill string. The system generates pressure pulses in the drilling fluid or mud, typically by way of a valve or siren type of device. The pulses are detected at the surface by suitable means, e.g., pressure sensors, strain gages, accelerometers, and the like, which are in general directly attached to the drill string or the stand pipe.
Borehole telemetry is a well established technology. Improvements to this technology as have been made over the past decades are published for example in a large number of patents, including U.S. Pat. Nos. 3,790,930; 3,820,063; 4,739,325; 4,932,005.
Of particular interest for the scope of the present invention are the numerous attempts being made to improve the data detection of the transmitted data at the surface. It should be noted that the drilling process presents an exceedingly noisy environment for telemetry owing to the mechanical generation of broadband noise and to the drilling fluid circulation system.
To improve the signal-to-noise ratio, the data as gathered by the sensor units can be encoded such that the distortion by noise has less impact on the data recovery. Usually employed encoding schemes include Frequency Shift Keying (FSK), Phase Shift Keying (PSK) or m-ary pulse coding. Alternatively a binary non return to zero coding may be used. Different encoding methods are described for example in U.S. Pat. Nos. 3,789,355 and 4,562,559.
In U.S. Pat. No. 5,381,092 the signals from those sensors which evaluate the earth formation are subdivided prior to transmission into a plurality of groups, each group represented by one value.
In U.S. Pat. No. 5,055,837 an attempt is described to improve the quality of the transmission by determining a transfer function which characterizes the transmission properties of the drilling fluid column in the drill pipe.
In an acoustic telemetry system, as described in U.S. Pat. No. 5,128,901, the data signals are (pre-)conditioned to counteract distortions caused by the drill string.
A filtering technique to cancel or minimize noise in the transmitted data signals is disclosed in U.S. Pat. No. 4,878,206. This known approach uses independent measurements of the vibrations of the drill string at the surface to remove pressure disturbance caused by these vibrations and affecting the mud column pressure. A similar technique is known from U.S. Pat. No. 5,289,354.
The specific problem of bit synchronization is described for example in U.S. Pat. No. 4,001,775. Each bit is represented by a change in phase of an acoustic signal. In addition to this, each bit, i.e. each phase shift, is transmitted over a predetermined number of cycles generated by a reference clock.
A combinatorial solution for en- and decoding of MWD signals is known from U.S. Pat. No. 4,908,804. Each datum is transformed prior to transmission into one of a combinatorial set of a number of nominally identical pulses distributed over a larger number of subintervals of a fixed time interval.
The Bayesian theory to discern different hypothesis when given experimental evidence (data) has been attributed to Rev. Thomas Bayes, who first discovered it back in 1763. A modern summary of Bayesian theory is presented for example by E. T. Jaynes, in an article titled "Confidence Intervals versus Bayesian Intervals", which is published in: "Papers on Probability, Statistics and Statistical Physics", R. D. Rosenkrantz (Ed.), Kluwer, 1983, pp. 149-209.
A possible application of this theory to telemetry is described in a conference paper by C. S. Christensen, titled "An algorithm for telemetry decommutation using Bayesian decisions", published 1970 in: Proceedings of the 3rd Hawaii international conference on system science, B. S. M. Granborg (Ed.), pp. 822-4, by Western Periodicals Co., Hollywood, Calif., USA. The author applies a Bayesian decision algorithm to the received and demodulated string of bits in order to eliminate bit errors and to associate the corrected bits with one of several telemetry channels. This and similar methods have apparently been used when receiving signals transmitted from a spacecraft, such as launched by the National Aeronautics and Space Administration (NASA) in their Mariner and Voyager deep space exploration program.
To appreciate the scope of the present invention in the light of this prior art, it is important to note that Christensen does not attempt to solve the "demodulation" problem, i.e. the problem to translate the analog signal into a string of bits
In view of the above cited prior art it is an object of the invention to provide an improved method and apparatus for demodulating an analog telemetry signal into a digital data signal. It is a further object of the invention to provide a drilling telemetry system with improved signal recognition. The system should be compatible with or independent from the various transmission media and encoding methods. It is a particular object of the invention to provide such a system for mud pulse telemetry in the low frequency domain.